1. Field of the Invention
This invention relates to a dispersal signal removing apparatus which can remove a dispersal signal from a reception signal which has a dispersal signal component therein.
2. Description of the Relates Art
A signal such as, for example, a video signal which is transmitted by way of a broadcasting satellite or a communication satellite is normally superimposed with an energy dispersal signal (hereinafter referred to merely as dispersal signal) of a triangular waveform in order to disperse the energy of satellite radio waves so that it may not concentrate upon a particular frequency or frequencies within a transmission band. If a dispersal signal remains on a video signal, then it will appear as a plane flicker on a screen. Therefore, it is necessary to remove, upon reception of a video signal, a dispersal signal from the video signal.
A dispersal signal is conventionally removed using a clamp circuit or using both of a clamp circuit and a dispersal canceler. Means for removing a dispersal signal is described first.
When, for example, a MUSE signal is to be broadcast using a broadcasting satellite, such a dispersal signal as shown by a curve (b) in FIG. 8 is superimposed on a MUSE signal shown by another curve (a) in FIG. 8. The dispersal signal is a triangular wave signal which presents its lowest level at the center of a first field of the MUSE signal and presents its highest level at the center of a second field of the MUSE signal, and the difference in frequency between the black and white levels when the MUSE signal is in an FM modulated form is 10.2 MHz while the peak-to-peak difference in frequency of the dispersal signal is 600 KHz. Consequently. the energy dispersal amount is 5.9%.
Meanwhile, a dispersal signal when the MUSE signal is transmitted using a communication satellite is shown by a curve (d) in FIG. 8. In particular, while the difference in frequency between the black and white levels when the MUSE signal is in an FM modulated form is 8.5 MHz, the peak-to-peak difference in frequency of the dispersal signal is 2.2 MHz. Consequently, the energy dispersal amount is 26%.
Thus, when a dispersal signal having such a small amplitude as seen from the curve (b) of FIG. 8 is superimposed on the MUSE signal, the dispersal signal is normally removed using such a clamp circuit 6 as shown in FIG. 9. Referring to FIG. 9, an FM modulated signal of a MUSE signal overlapped with a dispersal signal is first demodulated by an FM demodulator not shown and then supplied from an input terminal 1 to the clamp circuit 6 by way of a buffer amplifier 2. The output of the clamp circuit 6 is supplied to an analog to digital (A/D) converter 12 by way of a buffer amplifier 5, and the output of the analog to digital converter 12 is supplied from an output terminal 13 to a decoder not shown.
The clamp circuit 6 includes a capacitor 7 interposed in series between the buffer amplifiers 2 and 5, and a series circuit of a resistor 8 and an on/off switch 9 connected between a junction between the capacitor 7 and the buffer amplifier 5 and the input terminal 10 to which a clamp voltage which has a level equal to one half a maximum level of the video signal is supplied. The on/off switch 9 is controlled so that it presents an on-state for a period of time of the sample numbers 3 to 9 within a period of a horizontal synchronizing signal (for a HD period) of a MUSE signal shown in FIG. 10 but presents an off-state for any other period of time.
A residual signal which is the output of the clamp circuit 6 when the amplitude of the dispersal signal superimposed on the MUSE signal is 600 KHz in frequency difference upon FM modulation as shown by the curve (b) of FIG. 8 is shown by a curve (c) in FIG. 8. The amplitude of the residual signal corresponds to -52 dB or so when the amplitude from the black level to the white level is 0 dB, and accordingly, such a noise as a plane flicker is not produced on a reproduced image due to a dispersal signal removing action of the clamp circuit 6.
It is to be noted that, when the slope of the dispersal signal x is represented by K as seen from a curve A of FIG. 11, the capacitance of the capacitor 7 of the clamp circuit 6 by C and the resistance of the resistor 8 by R, the residual signal y shown by another curve B in FIG. 11 is given by EQU y=CRK{1-exp(-t/CR)}
On the other hand, when the dispersal signal has a great amplitude (2.2 MHz) as seen from the curve (d) in FIG. 8, the residual signal which is the output of the clamp circuit 6 presents such a waveform as shown by a curve (e) in FIG. 8. The residual signal corresponds to -38 dB or so when the amplitude from the black level to the white level is 0 dB, and accordingly, the residual signal makes a flicker noise which has an influence upon a reproduced image.
When the dispersal signal level is high to a certain degree, a dispersal canceler is used together with a clamp circuit as shown in FIG. 12 in order to remove the dispersal signal.
Referring to FIG. 12, an FM modulated signal of a MUSE signal superimposed with a dispersal signal is demodulated by an FM demodulator not shown and is inputted from an input terminal 1 by way of a buffer amplifier 2 to a dispersal canceler 3 which is formed from a pair of resistors 3A and 3B. The output of the dispersal canceler 3 is inputted to a clamp circuit 6 similar to the clamp circuit 6 shown in FIG. 9 by way of a buffer amplifier 5. The output of the clamp circuit 6 is supplied by way of another buffer amplifier 11 to and digitally coded by an analog to digital converter 12, the output of which is supplied from an output terminal 13 to a decoder not shown.
A cancel signal generating circuit 24 is connected to an end of the resistor 3B of the dispersal canceler 3 remote from the end at which the resistor 3B is connected to a junction between the other resistor 3A and the buffer amplifier 5. The cancel signal generating circuit 24 is constituted from a timing signal generator 34 and an integrating circuit 27 to which a timing signal is supplied from the timing signal generator 34. The integrating circuit 27 is constituted from an operational amplifier 28 having its non-negated input terminal grounded, an input side resistor 29 connected to the negated input terminal of the operational amplifier 28, and a capacitor 30 connected between the output terminal and the negated input terminal of the operational amplifier 28.
In the cancel signal generating circuit 24, a timing signal of a rectangular waveform the phase of which is reversed for each field is generated from the timing signal generator 34 and is supplied to the integrating circuit 27, at which it is integrated to generate a cancel signal which is opposite in phase to the dispersal signal of the triangular waveform described above. The cancel signal thus generated is supplied to the resistor 3B of the dispersal canceler 3. Consequently, the dispersal signal of the input signal is removed completely or attenuated.
While the conventional means for removing a dispersal signal either includes a clamp circuit or includes both of a clamp circuit and a dispersal canceler in this manner, a dispersal signal having a great amplitude cannot be removed effectively by the means which only includes a clamp circuit.
On the other hand, the means including both of a clamp circuit and a dispersal canceler cannot cope with a reception signal having a dispersal signal component having a different frequency or amplitude because the cancel signal is fixed, and accordingly, it has a problem that it is necessary to vary the frequency or amplitude of a cancel signal to be generated, for example, by manual operation depending upon a type of a reception signal.
For example, in the high definition television broadcasting which makes use of a broadcasting satellite, the dispersal signal is a triangular wave signal of 30 Hz and the energy dispersal amount is 5.9%, but in the NTSC broadcasting which employs a broadcasting satellite, the dispersal signal is a triangular wave signal of 15 Hz and the energy dispersal amount is 3.5%. Further, in the high definition television broadcasting which makes use of a communication satellite, the dispersal signal is a triangular wave signal of 30 Hz and the energy dispersal amount is 11 to 26% or so.
Thus, a dispersal signal removing apparatus is demanded which can automatically correspond to any of various dispersal signals and remove or attenuate the dispersal signal at least to such a degree at which a flicker noise on a screen is eliminated.